The reproductive state of lactation, which occurs in all mammals, is associated with an inhibition of reproductive cyclicity and ovulation due to a suppression of gonadotropin releasing hormone (GnRH), the hypothalamic neuroendocrine neurons regulating reproduction. The focus of our studies is to identify the afferent neuronal pathways activated during lactation by the suckling stimulus that are responsible for the suppression of GnRH neuronal function. This past year, we have used an acute resuckling model to make the following observations 1) During the first 3 hours after reapplying the suckling stimulus, novel expression of Neuropeptide Y (NPY) mRNA and peptide is induced in cells located in the area of the dorsal medial hypothalamic nucleus. In addition, NPY gene expression is increased in the arcuate nucleus, reaching maximum levels by 24 hours of resuckling. Using both anterograde and retrograde tracers, we have evidence that NPY neurons from these areas project to the paraventricular nucleus, suggesting that these neurons may be involved in regulating the increase in food intake associated with lactation and in the inhibition of GnRH neuronal activity. 2) Using cFos expression as a marker of neuronal activation, we have identified several specific areas within the brainstem that are activated by the suckling stimulus; these are the ventrolateral medulla, locus coeruleus, parabrachial nucleus and periaqueductal grey. These areas all send projections to the hypothalamus suggesting they may be potential candidates for relay of signals arising from suckling to the hypothalamus. An understanding of the mechanisms by which the suckling stimulus imposes an inhibition on GnRH neuronal function provides information that is relevant to primates (including humans), in which the reproductive neuroendocrine axis regulating ovarian cyclicity is also inhibited. These studies have relevance to women's reproductive health as they will increase our understanding of hypothalamic causes of infertility and provide new approaches for contraception.